IF we do come across poachers, then for your own safety, you will have to take a firm back seat, stay right back and leave us to it,” says one of my guides for the evening, Environment Agency officer Paul Newman.

Since Paul, 44, and his colleague Cormac Meenehan, 35, are wearing body armour, carrying batons, and have the power to arrest, this arrangement seems a sensible one.

Evidently, patrolling Hampshire’s rivers is a serious business. It is also big business.

A recent report from The Test and Itchen Association put the value to the local economy of returning salmon stock at somewhere around £3.7m.

In 2011 there were estimated to be 1,677 salmon in the Itchen and the Test, giving each fish a value to the economy of more than £2,000.

“Wild salmon and sea trout will fetch about £10 a lb on the black market,” Paul explains quietly as we work our way along a section of the River Wallington.

The fish may fetch an attractive price, but penalties can be severe and poaching salmon or sea trout can carry a fine of up to £50,000.

The agency says that some rivers in Hampshire are considered to have “marginal and unsustainable salmon and sea trout populations”, and illegal fishing threatens their long-term survival.

It is estimated that about 10 per cent of the fish population is lost to poachers, though officers admit the actual figure could be much higher.

“We probably see only the tip of the iceberg,” Paul says.

It is the dead of night as we start an eight-hour night time patrol that will take in the rivers Itchen, Hamble and Test, as well as a small area of coast near Lee-on-Solent.

We stop near a sea trout lie as Paul explains that this spot — the exact location of which he asks me not to disclose — is where the fish wait until it rains and they are at their most vulnerable.

As we talk I notice Cormac scanning the bank with his torch, looking for tell-tale signs of activity and I am reminded of hunters tracking their prey.

“Anglers, or I should say poachers, are messy. We’re looking for bits of reel, bait, any signs of activity, really,” Cormac tells me.

Another constant worry for them is the conservation limit — the minimum number of eggs that need to be deposited in order to sustain the population.

Paul, who has been in the job for 13 years, explains: “In the Itchen we’re only getting somewhere in the region of 700 salmon returning a year. The Itchen never hits its conservation limit.”

Both men are keen anglers who like to spend their spare time fishing with friends. The agency’s official aim is to ‘conserve and maintain the diversity and population’ of the fish, but one senses that a personal love affair with these rivers plays no small part in the duo’s dedication to the job.

But enforcement is just one aspect of that job, as their qualifications and extensive knowledge of fisheries and waterways means they are always on hand to advise local fisheries on handling contaminations and maintaining a healthy stock.

As Cormac puts it: “Everything from handling planning consents to getting kids in to fishing. It’s a wide and varied role.”

He says that much of their work is intelligence-led and they are always on the lookout for tip-offs from anglers, or reports from local restaurants of people attempting to sell salmon ‘through the back door’.

Clearly his knowledge and love of fishing leaves him well-placed to build contacts amongst the angling community.

We are an hour in to the patrol when we pull up near Lee-on-Solent, encountering a handful of legitimate night time anglers. As Paul looks out to sea with his thermal imaging device, Cormac mingles easily among them, gathering snippets of intelligence here and there.

Before we push on to the next check point, Cormac confides: “I do this job because I love it. Coming from Ireland where legislation is not at the same stage as the UK, you can see that the rivers are becoming less abundant with fish.

“We’re trying to do this to secure a healthy level of fish so that our children can enjoy it.”

Technology plays a key role in this battle — on both sides of the law.

Agency officers are equipped with thousands of pounds-worth of equipment, including night vision scopes and hi-tech binoculars. But the poachers are responding in kind, presenting an ever-evolving challenge.

“The gear these guys use nowadays has moved on. They use gill nets that used to have corks on that we could see in the water. The nets now you buy online with tiny, tiny corks that we can’t even see, and they can fit them in their pocket.

“And they can get night vision on the internet for as little as £100,” says Paul.

Poachers themselves range from foreign nationals attempting to make money from the sale of the fish, to country folk for whom poaching is something of a tradition, passed down from generation to generation.

In this light, I wonder if the officers have any sympathy for the men they’re tracking.

“No, none,” Cormac says.

“Because, you can buy salmon in the shops now and enjoy it. But if people keep poaching fish like this there’ll be nothing left in these rivers for the future.”